The present invention relates to a compressed air production facility for compressing gas such as air, and in particular, to a compressed air production facility in which a variable speed compressor adapted to change its rotational speed so as to adjust its capacity, and a constant speed compressor are operated in parallel.
For example, a compressed air production facility which is capacitively controlled, is disclosed in Japanese Laid-Open Patent No. H9-250485. The compressor disclosed in this document is provided in its compressed air outlet port with a pressure sensor which detects a discharge pressure. With this arrangement, PID control is carried out for the rotational speed of the compressor with the use of the thus detected discharge pressure in order to capacitively control the compressor.
Conventionally, in such an arrangement that a plurality of compressors which are connected with one another in parallel, are operated, load pressures (start pressures) and unload pressures (rest pressures) of the compressors are set to be different from one another bit by bit. In this case, a change in power consumption (%) with respect to an air consumption (%) by a compressor is such that the power consumption is linearly decreased, as the air consumption is decreased, in either a suction throttling system (S1) or a combination system (S2) in which a system for repeating load operation and unload operation by reducing the discharge pressure is combined with a suction throttling system. However, about 30% (in the case of the system S1) or 70% (in the case of the system S2) of the rated power is consumed even though the air consumption becomes zero.
If a plurality, for example, two, of compressors having the above-mentioned characteristic are automatically operated in parallel and stopped, characteristics S1, S2 shown in FIG. 2, are exhibited. As clearly understood from this figure, the operation of two compressors having volumes which are substantially equal to each other is advantageous in view of power saving in comparison with operation of a single compressor having a discharge air volume which is twice as large as that of the former two compressors.
On the contrary, in a system (R1) using the above-mentioned variable speed compressor disclosed in the above-mentioned Japanese Laid-Open Patent No. H9-250485, a power consumption (%) with respect to an air consumption (%) has a characteristic which is substantially exhibited by a direct proportional curve, but is deviated slightly therefrom if the air consumption is excessively low. Further, if the air consumption becomes zero, the power consumption is lowered to about 10% of its rated power. Accordingly, in the case of operation of two compressors each having this characteristic in parallel, a characteristic R1 shown in FIG. 2 can be obtained, which is excellent in power saving in comparison with those of the systems S1, S2. Although the variable speed compressor has this advantageous characteristic, a variable speed mechanism such as an inverter is required so as to cause such a disadvantage that the costs of the facility becomes higher.